The present invention relates to a ventricular-assist method and apparatus and, more particularly, to a ventricular-assist device (VAD, which utilizes only a single cannula and can assist both the acute and chronic failing heart. The device can be used as bridge to recovery of the failing heart, as a permanent implanted assist device or as a bridge to transplantation or as a bridge to other Cardiac Assist Devices. The invention also relates to a method of sustaining the failing heart utilizing a ventricular-assist device and an algorithm for operating a ventricular-assist device.
The normal range of cardiac output, normalized to total body surface, is wide, 2.5 to 3.8 liter per minute per one square meter (1/min/m2). In general, cardiac assist is necessary whenever a patient""s cardiac output drops below the adequate blood supply needed to sustain proper blood perfusion, which is around 2 1/min/m2. Failure of the cardiac ventricle to contract and to eject the blood out of the ventricle and thereby to supply adequate flow is defined asxe2x80x9csystolic failurexe2x80x9d. However, more than 50% of the patients over 60 display inadequate ventricle filling and tissue congestion, which is defined as xe2x80x9cdiastolic failurexe2x80x9d. Cardiac assist is used to treat patients suffering from heart failure at a stage where conventional drug therapy proves ineffective.
Cardiovascular diseases (CVD) represent the leading cause of death in the industrialized world. CVD claimed 960,592 lives in the US in 1995 (41.5% of all deaths for that year).
Congestive Heart Failure (CHF) is a chronic disorder that develops over time, manifested clinically by an enlarged heart and symptoms and signs of low cardiac output and tissue congestion. The low cardiac output leads to decreased blood perfusion to vital organs (liver, kidney and brain). The CHF is also characterized by lung congestion (recurrent pulmonary edema), which threatens life and requires frequent hospitalization. CHF is associated with profound symptoms that limit daily activities, is a debilitating disease with poor quality of life. CHF is the most common cause of hospitalization of patients over 60 years of age.
According to the US National Heart Lung and Blood Institute (NHLBI) and the American Heart Association there are approximately 5 million patients who suffer from Congestive Heart Failure (CHF) in the US and between 400,000 and 500,000 newly diagnosed patients each year. Long-term survival rates are low and the 5 year mortality rate for patients with CHF is 75% in men and 62% in women, while in patients with decompensated heart failure the mortality rate is 60% per year.
CHF has various etiologies, including cardiovascular disease (diseases which affect blood flow to the myocardium), chronic hypertension (high blood pressure), incompetent valves, inflammation of the heart muscle or the valves, substance (amyloid) accumulation and congenital heart problems.
Patients suffering from Congestive Heart Failure (CHF) are initially treated with medication. While conventional drug therapy may delay the progress of CHF, it is not curative. Cardiologic intervention (such as Angioplasty and Stenting), surgery (Heart by-pass surgery, Cardiomyoplasty, Partial Ventriculectomy known as Batista""s procedure), and mechanical devices are often considered when drug therapies prove ineffective or inadequate. Electrical disturbances of the heart that threaten or impair the quality of the patient""s life have been treated effectively with pacemakers and implantable defibrillators. However, congestive heart failure has not been addressed effectively. Currently, the only available method of treating end-stage CHF is a heart transplant.
The demand for temporary and permanent cardiac-assist devices for the treatment of chronic heart failure is remarkably large; in 1993 between 40,000 to 70,000 patients needed life-sustaining assist devices or a total artificial heart, and an additional 80,000 to 200,00 patients needed quality of life improvements by surgery (Cardiomyoplasty or Heart Booster).
Ventricular-assist devices are needed for:
1. Bridge-to Transplantationxe2x80x94patients awaiting heart transplants and who are not scheduled and when the heart failure is unresponsive to medical treatment.
2. Bridge-to-Recoveryxe2x80x94cardiac assist for patients whose heart has sustained serious injury, but can recover if adequately supported. This includes the use of a cardiac-assist device after open heart surgery in order to provide support until the heart regains its ability to pump, and severe myocardial infarction refractory to conventional treatment with medication. Temporary cardiac support is intended primarily to:
a) Prevent or reduce damage to vital organs (brain, kidneys) from cardiac failure and to support adequate blood circulation.
b) Allow the failing heart to recover, i.e. to provide adequate coronary perfusion to the heart itself.
c) Reduce the energy consumption of the failing heart and to improve the balance between energy supply and demands.
3. Permanent support for the failing heart, for patients that are not candidates for heart transplantation.
Existing temporary mechanical cardiac devices are divided into three groups:
1. Temporary cardiac assist for several hours, as the intra-aortic balloon that is frequently utilized for patients with heart failure after open-heart surgery, due to failure to wean from the cardiopulmonary bypass.
2. Long-term (days, weeks, months) Ventricular Assist Device (VAD), as a bridge to heart transplantation or a bridge to recovery.
3. Permanent support by permanent VAD or by Total Artificial Heart (TAH).
Intra Aortic Balloon Pump (IABP). The IABP has been in clinical use for over 30 years. The IABP consists of a balloon (30-50 ml) that is inserted into the descending aorta and is inflated during the diastole and deflated during the systole. The IABP increases the cardiac output by less than 0.5 1/min/m2. Consequently, although it was designed to assist a failing heart by improving blood perfusion, it requires a certain threshold level of cardiac output and cannot take over the pumping function of the heart. As a result, it can only be utilized in treatment of patients who require mild levels of mechanical assistance (unless there is a supplemental assisting heart device).
The main advantages of the IABP are that it increases the coronary flow and decreases the afterload (the work against which the ventricle works). Hence the device improves the energy supply to the myocard, reduces the energy consumption and allows the heart to recover. However, the IABP is used only for short-term circulatory assist due to high risk of severe thromboembolic complications.
Ventricular Assist Devices (VAD)xe2x80x94VADs take over the complete pumping function of one or both sides of a failing heart. They unload the assisted ventricle. Left Ventricular Assist Devices have been approved for use by the FDA as bridge-to-heart transplantation, to keep alive those awaiting a donor heart. These devices have also been approved by the FDA for use by patients whose hearts are in failure but may be able to recover by reducing the myocardial work (unloading), including patients in postsurgical life-threatening heart failure.
More than a dozen companies are developing devices, ranging from left-ventricular assist products to total artificial hearts, that offer CHF patients either longer-term support with an alleviation of symptoms, and/or an alternative to heart transplant. Some of these (Thermo CardioSystems, Thortec, Abiomed and Baxter Healthcare) have ventricular assist products on the U.S. market. Ventricular-assist devices are generally employed on a temporary basis, with treatment periods ranging from a few hours to a few weeks, or at most, a limited number of months. However, some devices have been designed for long-term use and can be considered lifetime support systems. However, to date, such lifetime support is still in developmental and experimental stages and has not been approved by the FDA.
The currently available assist devices can be sorted by the following major three criteria:
1. Mode of operation:
a. Bypass circulation.
b. Direct mechanical actuator.
2. Type of flow: Pulsatile or non-pulsatile flow.
3. Location: implanted devices or extra-corporeal devices.
Most of the available VADs in the market belong to the bypass group. There are four major types of these VADs: Roller pumps, Centrifugal pumps, Pneumatic devices and Electrical devices. These devices differ in design, indications and duration.
Roller and Centrifugal Pumps are approved for short-term (i.e. hours) support of patients undergoing heart surgery. These devices generate a non-pulsatile blood flow which severely restricts the time patients can safely remain on support. They also require additional medical personnel to provide constant monitoring and ensure that the pump is operating correctly. Recently, new centrifuge pumps are being developed that are highly a efficient (low energy consumption) and can be implanted for prolonged assist.
The pneumatic devices can provide full circulatory assistance and were the first to be approved for clinical use. The BVS 5000, developed and manufactured by Abiomed Inc. was also approved by the US FDA as a bridge-to-recovery device for the treatment of reversible heart failure. The BVS-5000 (BVS) is a pneumatic extra-corporeal, bi-ventricular assist device, allowing the heart to rest and recover its function. However, the blood circulates out of the body and the patient cannot be ambulatory. The company""s first full year of marketing the BVS in the US was 1994.
Thoratec Laboratories Corporation has developed an implantable pneumatic-assist device, which is connected to an external-drive by a percutaneous air-drive line. This system was also approved by the FDA as a bridge to heart transplant.
The electrical VAD are completely implantable with an implantable controller, battery and charger (secondary coil). The main electrical pulsatile implantable pumps are: Novacor N-100 (Baxter Healthcare Corp.), Heartmate 1000 NE LVAS (ThermoCardioSystem Inc.) and Pennsylvania State University System.
In September 1998, the first two ambulatory implantable left ventricular-assist systems (LVAS), from Baxter and ThermoCardioSystem Inc (TCS), were approved in the U.S. TCS"" implantable electric HeartMate LVAS has been marketed since 1994. In Europe, the Baxter Novacor LVAS has been approved as a commercial product since 1994. These devices represent a significant advance over first-generation technology, since they allow patients to live outside the hospital while awaiting transplantation. The Baxter Novacor is an electromechanical pump that is implanted in a patient""s abdomen and connected to the left ventricle of the heart. The system is operated by an external, portable electronic controller, and is powered by battery packs, which the patient typically wears around the waist in a shoulder vest or backpack. Nearly 900 patients worldwide have received the Novacor LVAS: two patients have currently been supported for more than three years by their original devices. In Europe, the device has helped to rehabilitate some patients"" hearts to the extent that neither VAD assistance, nor heart transplant were necessary.
Transplant bridging, and possibly long-term cardiac assistance may also be accomplished with implantable axial flow and centrifugal pumps. An axial flow VAD, that includes a high-speed rotor, has been recently developed by Micromed in co-development with the National Aeronautics and Space Administration (NASA). This miniaturized DeBakey/Ventricular Assist Device (30 mmxc3x9776 mm) weighs only 93 grams, making it about one-tenth the size of portable heart-assist devices already on the market.
Examples of companies pursuing cardiac-pumping technology include: Jarvik Research, Medtronic Inc., 3M Corporation Inc., Kirton Medical, Micromed Technology and Cardiac Assist Technologies.
Direct Mechanical Actuator
Unlike all the above VAD""s that pump the blood out of the ventricle into the aorta and bypass the failing heart, the Direct Mechanical Actuator proposes a different approach, taken by Cardio Technologies. This company is pursuing a cuff-like device that is placed around the outside of the heart. This device applies external pressure to enhance blood flow. A somewhat similar device, designed to reduce the size of an enlarged heart, is under development by Acorn Cardiovascular. Abiomed was also involved in some early development stages of the Heart Booster system designed to wrap around the heart, to provide ventricular augmentation.
Alternative Surgical modalities
Three additional surgical methods have been developed recently as alternatives to cardiac assist, in order to improve the residual cardiac function: 1) Dynamic Cardiomyoplasty; 2) Partial Ventriculectomy or Batista operation, and 3) Percutaneous transmyocardial revascularization (PTMR). However, these methods are controversial.
In the Dynamic Cardiomyoplasty technique, a surgeon wraps some of the patient""s skeletal muscle around the weakened heart and stimulates the repositioned muscle to synchronously squeeze the heart during systole. Dynamic Cardiomyoplasty is highly invasive and involves complicated surgical procedures. Medtronic is also involved in clinical studies of this pacemaker-aided technique using the latissimus dorsi muscle. Percutaneous transmyocardial revascularization (PTMR) is a recently approved catheter-based laser technique that involves drilling about 50 tiny holes in the left ventricle to improve blood flow to the heart muscle. This laser surgery was suggested as a cost-effective alternative to transplantation for certain patients with severe angina, who were not candidates for angioplasty or bypass surgery. The precise mechanism underlying this approach is controversial. Moreover, the efficacy of this method is under investigation.
It is the principal object of the present invention to provide an improved ventricular-assist device that, has the following advantages over the current available VADs:
a. Simple Implantation. It allows cannulation and implantation by minimal invasive approach. This will shorten the surgery and the postoperative recovery and hospitalization for rehabilitation.
b. Allows bridge to recovery of the failing heart, i.e. it should improve the viability and function of the residual cardiac tissue of a failing heart. The mode of operation is based on the physiological characteristics of the cardiac muscle, so that it will allow the failing heart to recover by the augmentation of the coronary perfusion and by decreasing the energy consumption and supporting the cyclic heart function (decreasing the end diastolic volume while increasing the systolic pressure and the stoke volume). The device should be free from the drawbacks of bypass (shunting) devices which unload the failing heart and may lead to atrophy, endocardial ischemia and right heart failure.
c. Economical energetically, so that it allows prolonged support on rechargeable portable or implanted power supply.
d. No valves are requiredxe2x80x94which allow prolonged durability.
e. Utilizes only a single short conduit, thereby decreasing the likelihood of thrombo-embolic complications.
f. Imposes few physiological shear stresses on the blood, and hence avoids the complication of hemolysis, encountered in the axial flow devices.
It is also an object of the invention to provide an improved method of assisting a failing heart while attaining some or all of the aforementioned advantages.
Still a further object of the invention is to provide a method of and an apparatus for ventricular assistance whereby drawbacks of earlier systems can be avoided. The assistance provided can be more reliable and the energy drain on the assisted heart can be minimized.
These objects are attained, in accordance with the invention in a ventricular-assist method, which comprises:
(a) inserting into the failing ventricular cavity (left, right or both) of a failing heart through a wall thereof a respective cannula connected to a blood displacement chamber and actuator;
(b) in cadence with normal functioning of the failing heart, effecting flow of blood into the failing ventricular cavity (inflow), generated by the blood displacement actuator, with each heart beat. The inflow from the displacement chamber commences only after opening of an outlet valve of the respective ventricular cavity of the failing heart or only after a detected shortening of a monitored region of a wall of the respective failing ventricle, and continues during an ejection phase of the respective ventricle, thereby augmenting ejection volume (stroke volume) from the respective ventricular cavity by up to a maximum volume of blood inflow, into the respective ventricle, per systolic phase;
(c) controlling a time course of blood flow through the cannula, generated by the blood displacement actuator, into the ventricular chamber (inflow) in step (b) to reduce the shortening of a respective ventricular wall of the failing heart by comparison with ventricular wall shortening while the device does not provide assist (inflow), and to prevent ventricular wall stretching (eccentric work). (Controlling the inflow time course allows augmentation of the systolic pressure within the respective failing ventricle);
(d) controlling an increase in the total ejected volume out of the ventricular outlet with the blood displacement actuator. (The ventricular outflow is the sum of contribution of the ventricular wall shortening and the VAD inflow into the ventricle);
(e) retracting blood from the ventricular chamber through the same cannula (outflow), immediately upon closing of a respective outlet valve of the failing ventricle.
The method of the invention further comprises the steps of:
monitoring ventricular wall motion and the intraventricular pressure during blood flow into the ventricular chamber (inflow), generated by the blood displacement actuator, through the cannula in steps (b, c); and
controlling a profile of blood flow into the ventricle thereof to decrease the measured ventricular wall motion thereby obtaining an increase in the pressure within the respective cavity and increasing the cardiac output.
According to the invention, at least one parameter of ventricular wall shortening and at least one parameter of ventricle output can be measured during the cardiac cycle and in response to measurement of these parameters, selectively either in real time or by beat-by-beat computation, blood flow into the ventricle chamber and out of the ventricle chamber are controlled by the blood displacement actuator, to correspond to the desired cardiac output (ejected volume)and profile of blood flow.
The parameters of wall shortening which can be monitored are the ventricular volume, ventricular diameters, and ventricular wall strains or the ventricular out-flow in preferred embodiments of the invention.
The cannula is connected on the opposite side of the cardiac wall insertion to the blood displacement chamber, which is connected to the actuator with a computer-controlled pushing (inflow) and retracting (outflow) blood mechanism, into and out of ventricular cavity through the cannula inserted into the failing ventricular cavity (right, left, or both).
In another aspect of the method, the following steps are carried out:
(a) inserting into a failing ventricular cavity (left, right or both) of a failing heart through the cardiac wall thereof a respective cannula;
(b) in a cadence with normal functioning of the failing heart, generating inward blood flow (inflow) with the blood displacement actuator, through the respective inserted cannula with each heart beat. The inward blood flow commences only after opening of an outlet valve of the respective ventricular cavity of the failing heart or only after a detected shortening of a monitored region of a wall of the respective ventricular cavity of the failing heart and continues during an ejection phase of the respective ventricular cavity, thereby augmenting ejection volume from the respective ventricular cavity by up to a maximum of the blood flow volume through the respective inserted cannula into ventricular chamber per systolic phase;
(c) controlling a time course of blood flow through the respective inserted cannula into ventricle chamber in step (b) to reduce a shortening of a respective ventricular wall of the failing heart by comparison with ventricular wall shortening while the device does not assist the circulation (does not provide the inflow), and to prevent ventricle wall stretching (eccentric work);
(d) controlling an increase in the total ejected volume out of the ventricular outlet as defined by the sum of contribution of the ventricular wall shortening and the VAD inflow into the ventricle;
(e) retracting blood through the cannula from the ventricle (outflow) immediately upon closing of a respective outlet valve of the failing heart.
The apparatus can have a computer receiving input from the sensor and controlling the blood displacement actuator with an output. The computer is programmed for each heartbeat (n) to:
(a) evaluate cardiac output and work at the nth beat;
(b) compare the evaluated cardiac output and work at the nth beat with a desired cardiac output to determine an amplification factor (AF);
(c) multiply the amplification factor (AF) by a weighting function (W(t)) as determined by an operator to generate a magnitude of a feedback loop;
(d) evaluate ventricle wall shortening (Sn(t)) and compare the evaluated wall shortening with a desired wall shortening (Des(t)) to obtain a difference Errn(t)=Des(t)xe2x88x92Sn(t);
(e) generate the inflow function EXPn+1(t)=EXPNn(t)+AF*W (t)*Errn(t); and
(f) control the profile of the blood inflow into ventricle at a next beat (n+1).
The amplification factor (AF) is multiplied by a weighting factor (w(t)) at each beat where 0xe2x89xa6W(t)xe2x89xa61 and 0xe2x89xa6txe2x89xa6T, where t=0 is the time onset of the inflow and t=T is the end of ejection (systole).
Advantageously the computer is a computer, which controls the inward flow profile at the next beat (n+1) by regulating the onset time of the inflow and the profile function of inflow. The computer
a) calculates the desired profile of the inflow and outflow through the cannula, either in real time or by repetitive iterations and corrections, from beat to beat; and
b) regulates in real time, the timing of the inflow and outflow based on the monitored above sensors.
The computer can receive input from the sensor and can control the blood displacement actuator with an output. The computer being programmed for each heartbeat (n) to:
(a) evaluate cardiac output and work at the nth beat;
(b) compare the evaluated cardiac output and work at the n beat with a desired cardiac output and determine an amplification factor that will not cause ventricle wall stretch in part based upon additional inputs;
(c) evaluate ventricle wall shortening (Sn(t)) at the nth beat and providing the ventricle wall shortening as one of the additional inputs;
(d) detect possible ventricle wall lengthening from the evaluation of the wall shortening in step (c) and providing therewith another of the additional inputs, and triggering an alarm upon ventricular wall lengthening;
(f) determine a time course of blood flow from blood displacement actuator through the cannula into the ventricular chamber from the amplification factor and a desired profile of blood inflow; and
(g) generate a blood inflow function representing the time course of the control of the blood displacement actuator at a next beat (n+1) with the inflow function.
The means for detecting a state of the outlet valve can include at least one of the following:
a pressure sensor for measuring intraventricular pressure.
a pressure sensor for measuring aortic pressure or a gradient between intraventricular and aortic pressure;
ultrasound or electrical impedance means for measuring intraventricular volume or ventricle diameters;
a Doppler or an ultrasonic or electromagnetic flow meter measuring ventricle outlet flow;
strain gauges for measuring ventricle wall shortening;
a heart sound monitor; or
a cardiac electrical activity measuring device.